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tachymeter
The tachymeter (from Greek tachy ´ s = fast and
métron = measure) has a scale calibrated to
1 kilometer. It is used to measure speeds and
delivers results in kilometers or miles per hour.
Because speed is expressed as units of distance
per unit of time, it is necessarily dependent on
the measurement of time. Therefore, a chrono-
graph with a logarithmic tachymeter scale can
display speeds. It measures the time needed to
travel a known distance, and the scale shows how
fast the observer was moving.
telemeter
The telemeter scale is graduated in kilometers and
converts the elapsed time between a visual event
and its acoustic manifestation into the distance
between the observer and the observed event.
It takes into account the difference between the
speed of light (300,000 km/s) and the speed of
sound (333 m/s). Until the 1960s, even the military
calculated distances according to this method.
If it took five seconds for the observed muzzle
flash to be followed by the thunder of the can-
non, troops knew they were somewhat farther
than one and a half kilometers away from the gun
emplacement: 333 m/s × 5 s = 1665 m.
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